In the electronics industry, a continuing objective is to further and further reduce the size and weight of electronic devices while simultaneously increasing performance and speed. Cellular telephones, personal data devices, notebook computers, camcorders, and digital cameras are but a few of the consumer products that require and benefit from this ongoing miniaturization of sophisticated electronics.
Integrated circuit (“IC”) assemblies for such complex electronic systems typically have a large number of interconnected IC chips. The IC chips, commonly called dies, are usually made from a semiconductor material such as silicon or gallium arsenide. Photolithographic techniques are used to form the various semiconductor devices in multiple layers on the dies.
Dies are encapsulated in a molded plastic package that has connectors or leads on the exterior of the package that function as input/output terminals for the die inside the package. The package includes a leadframe, a die mounted on the leadframe, and wires connecting the die to the leadframe.
The die is conventionally mounted to the top surface of the leadframe with, for example, a layer of an adhesive or an adhesive film, and then electrically connected to the leadframe by a number of fine, conductive wires, typically gold (“Au”) or aluminum (“Al”). The wires are attached to the die at the bonding pads of the die, which are located around the periphery of the die.
After the wires are attached, the die, the leadframe, and the conductive wires are encapsulated in a mold compound, such as plastic or epoxy. The encapsulation protects the leadframe, the die, and the fine conductive wires from physical, electrical, moisture, and/or chemical damage.
The encapsulation process begins by placing the leadframe, the die, and the fine conductive wires in a mold. Next, a mold compound is injected into the mold. The mold compound flows through the mold, encasing the leadframe, the die, and the conductive wires.
Typically, a mold encapsulates multiple semiconductor devices at the same time. A two part mold mounted on a hydraulic press is generally used.
Initially the two halves of the mold are held apart. One or more lead frames containing semiconductor devices are placed in an open half of the mold. The hydraulic press is then actuated and the mold closed, forming a cavity around each semiconductor die. It is not unusual for a mold to contain hundreds of cavities. Each of these cavities is connected by one or more gates, channels, and runners to one or more softened plastic central reservoirs or pots. A worm screw or ram compresses the plastic so that it flows into the cavities. As soon as the plastic has hardened, the mold is opened and the molded plastic packages removed.
Frequently, moveable pins are built into the mold to align the mold parts, to hold the lead frames in a particular location during molding, or to provide automatic ejection of the encapsulated parts. Also, the mold may contain other moving parts such as variable gates, vents, and dams. Thus, molds for encapsulating electronic parts, particularly semiconductor parts, are often very complicated.
In order to push liquefied plastic from the reservoirs into the many cavities, it is frequently necessary to inject or transfer the plastic at very high pressures. If the mold halves fail to seal tightly against each other or against the lead frames, undesired or unintended crevices may be present therebetween. At such high pressures, the crevices fill with plastic during encapsulation, producing unwanted thin webs of plastic. These thin webs of plastic are referred to as “flash” and can result from such imperfect sealing of the mold. Before the encapsulated electronic devices can be used, this flash must be removed. This increases the cost of manufacture and is undesirable. Also, flash is a significant cause of mold wear, requires additional labor for mold cleaning between molding cycles, and increases mold down-time.
In order to minimize flash, great pains are generally taken to machine the mating surfaces of the mold halves flat and parallel where they are to seal. Usually, they are carefully inspected for planarity during manufacture and after installation in the press. Powerful hydraulic cylinders in the mold force the mold halves tightly against the lead frames and each other. However, the force that can be applied in an effort to seal the mold is limited, since excessive force causes coining of the lead frames and rapid mold wear. Despite these efforts, flash continues to occur, even in the most carefully fabricated molds. The larger the mold, the more severe the problem.
Thus, a need still remains for improved encapsulation methods for electronic devices that reduce size, reduce weight, and eliminate unwanted flash.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.